Arrays may use LEDs within individual pixel elements. Such arrays may be used in flat panel displays. One problem with such displays is that the individual LEDs in the display may degrade at different rates due to different amount of use and other factors. When the difference in luminescence efficiency of LEDs within neighboring pixels becomes greater than a certain threshold, such as 5%, the pixel non-uniformity becomes observable, and the display is considered aged.
An aged display as described above may be no longer usable even though all pixels may still be brightly luminescent due to the observable non-uniformities in brightness between pixels. Such differential aging of pixels limits the life of a such a display to only a few months, making the displays not commercially viable.
Accordingly, techniques for correction of brightness levels of pixels to compensate for such differential aging in such displays have been developed. For example, the displays may be operated in a constant brightness mode instead of a constant current or voltage mode. The brightness level of each LED pixel may be monitored, and the current level may be adjusted to maintain a constant and uniform brightness for each LED pixel. It has been proposed to monitor the brightness level through the use of photodetectors, which are typically made from silicon technology.
Such photodetectors can be fabricated on a silicon substrate, with the LED layers formed over the photodetectors. This fabrication involves costly processing steps. For example, the metallic layer contacting the LEDs to the photodetector backplane must be transparent so that light can reach the photodetectors. This may reduce the forward quantum efficiency of the photodetectors by as much as 50%. Further, a distinct photodetector must be fabricated to support each LED, since the distance between the LED layers and the photodetectors will be submicron scale. Also, with such an approach, the photodetector circuitry will have to share the precious area on the substrate with active matrix device drivers that also occupy space on the silicon backplane.